Generate the Vulkan shaders at build time (#2199)

Use `add_custom_command` and `add_custom_target` instead of
`execute_process` so the generation of the Vulkan shader is done at
build time and not configuration time.

Use `configure_file` instead of string replacement.

Fixes #2179

Signed-off-by: Ahmed Hesham <ahmed.hesham@arm.com>

test_conformance/vulkan/CMakeLists.txt

@@ -32,3 +32,5 @@ include_directories("../common/vulkan_wrapper")
 add_subdirectory(shaders)
 
 include(../CMakeCommon.txt)
+
+add_dependencies(${${MODULE_NAME}_OUT} vulkan_shaders)